1. Field of the Invention
The present invention relates to a movable contact element used for the operation panel of electronic devices such as mobile phones, and a panel switch formed using the movable contact element.
2. Background Art
In recent years, electronic devices such as mobile phones are being reduced in size and thickness. More and more such devices are having an operation panel with panel switches which is formed by pasting an insulating sheet with movable contact points onto the wiring board of the devices. Such an insulating sheet with movable contact points is called a movable contact element.
FIGS. 6 to 9 show a conventional movable contact element and a panel switch formed using the element.
FIG. 6 is a cross sectional view showing a main part of the conventional movable contact element, FIG. 7 is a bottom view showing a conventional base sheet; FIG. 8 is a cross sectional view showing a main part of a panel switch formed using the movable contact element, and FIG. 9 is an exploded perspective view showing the conventional panel switch.
As shown in FIGS. 6 and 7, movable contact element 6 includes base sheet 1 made of a flexible insulating resin. Base sheet 1 is approximately rectangular-shaped and is provided on its bottom surface with adhesive 2. Base sheet 1 is further provided with rectangular holes 1A in predetermined positions. Movable contact element 6 further includes a plurality of movable contact points 5 made of an elastic metal thin plate. Each movable contact point 5 has a convex dome shape, and the outer surface of the dome is bonded to the bottom surface of base sheet 1 by adhesive 2 applied on the bottom surface. Movable contact points 5 are arranged in predetermined positions in this manner so as to form movable contact element 6.
As shown in FIG. 7, adhesive 2 applied by screen printing or the like on the bottom surface of base sheet 1 consists of adhesive portions 3 dotted around. Through adhesive portions 3, non-adhesive portion 4 is spread like a net and reaches the edges of base sheet 1 and rectangular holes 1A in base sheet 1.
FIGS. 8 and 9 show the conventional panel switch formed using movable contact element 6.
An electronic device (unillustrated) includes wiring board 7, which is provided on its top surface with a plurality of fixed contact points 8. Each fixed contact point 8 consists of a pair of outer fixed contact point 8A and central fixed contact point 8B. Fixed contact points 8 are arranged corresponding to movable contact points 5 of movable contact element 6. The electronic device further includes LEDs 9 soldered in predetermined positions on wiring board 7 in such a manner as to correspond to rectangular holes 1A of base sheet 1.
Movable contact element 6 is bonded on wiring board 7 by adhesive 2 applied on the bottom surface of base sheet 1 as follows. Each movable contact point 5 is positioned so that its circumferential bottom edge is placed on the corresponding one of outer fixed contact points 8A, and that the top of its dome faces the corresponding one of central fixed contact point 8B with a spacing therebetween. Thus the panel switch includes a plurality of switches located corresponding to movable contact points 5. With movable contact element 6 bonded on wiring board 7, LEDs 9 are exposed from rectangular holes 1A of base sheet 1.
Adhesive 2 on the bottom surface of base sheet 1 is applied in the form of dots because of the following reasons. If adhesive is applied all over the bottom surface, the adhesion is to strong to smoothly position movable contact element 6 on wiring board 7 of the electronic device. Furthermore, adhesive 2 in the form of dots can be printed more easily onto base sheet 1, thereby securing the entire bottom surface of base sheet 1 to be bonded onto wiring board 7.
The operation of the panel switch will be described as follows. First suppose that the user presses somewhere on base sheet 1 that corresponds to the top of the dome of one of movable contact points 5. The dome of movable contact point 5 is pressed via base sheet 1. When the pressing force of the user exceeds a predetermined level, the dome is deformed elastically to be turned upside down with a sense of moderation. Then the inner surface of the dome comes into contact with the opposed central fixed contact point 8B. As a result, the switch goes into the ON state where outer fixed contact point 8A and central fixed contact point 8B are electrically connected with each other via movable contact point 5.
When the user release the pressing force, the inner surface of the dome leaves central fixed contact point 8B so as to restore movable contact point 5 to the original convex dome shape with a sense of moderation. As a result, the switch returns to the OFF state where outer fixed contact point 8A and central fixed contact point 8B are electrically isolated from each other.
LEDs 9 are turned on and off according to a signal generated when a specific switch is operated.
The bottom surface of base sheet 1 is generally covered with a separator before movable contact element 6 is bonded on wiring board 7. Separators are used to protect movable contact points 5 from dust and foreign matters during transportation and storage. The separator is made of a flexible insulating film having a release-processed top surface. A detailed description of the separator will be omitted here.
Prior art documents related to the invention of the present application include patent document 1 (Japanese Patent Unexamined Publication No. 2002-203454) and patent document 2 (International Publication pamphlet WO 01/080263).
In the aforementioned conventional movable contact element and panel switch formed using the element, adhesive 2 on the bottom surface of base sheet 1 consists of adhesive portions 3 dotted around, and non-adhesive portion 4 is spread through adhesive portions 3 to let air in and out from the edges of base sheet 1. Therefore, repeatedly applying and releasing a pressing force to/from movable contact points 5 so as to repeatedly turn on and off the switches moves around the air contained in the domes of movable contact points 5. This allows air to move between non-adhesive portion 4 on the bottom surface of base sheet 1 and outside base sheet 1.
The air movement may cause fine foreign matters contained in the air to get into the contact area between movable contact points 5 and fixed contact points 8 through non-adhesive portion 4, thereby damaging the contact stability of the switches.